Thermal infrared sensors (also known as “infrared thermal pixels”, although this term is typically only used when an array of sensors is used) comprising a membrane (also known as “diaphragm”) suspended in a cavity by means of a plurality of beams (also known as “webs”) and having thermocouples arranged on said beams with one end located on the membrane and another end located on the substrate (also known as “bulk”), are known in the art.
The membrane is typically chosen as large as possible for maximizing infrared light reception, the beams must be mechanically sufficiently strong for carrying the membrane, and are typically chosen as long and as small as possible for minimizing heat loss from the membrane through the beams towards the substrate.
The working principle is as follows: an external object (or subject) emits IR radiation, which typically enters the cavity via a window or aperture in the package, and warms up the membrane, causing a temperature difference ΔT above the bulk temperature, which temperature difference is measured by means of a thermopile, i.e. a plurality of thermocouples connected in series. The thermopile provides a voltage indicative of the temperature difference.
Several different designs are described in the prior art. They can roughly be divided in two groups: a first group where the pressure in the cavity is “high-vacuum” (e.g. less than 100 mPa), and a second group where the pressure in the cavity is much higher, often referred to as “not high-vacuum” or “low vacuum”, typically having a pressure higher than 100 Pa (1 mbar). High vacuum devices typically provide larger signals, but require a more expensive sealing technique.
US2011/0174978 describes a thermal infrared sensor, some embodiments of which are replicated in this document as FIG. 1(a) to FIG. 1(d), illustrating some examples of thermal infrared sensors available on the market. Although several designs are shown for obtaining a good performance of the thermal sensor, further optimization still seems possible.
In view of the importance for thermal infrared sensors for having good detectability, there is still room for improvement.